1. Field of the Invention
The present invention relates to a TDMA Communicating Method and a TDMA receiving apparatus, in particular, to a TDMA communicating method of a base station that communicates with a plurality of zones that use different radio frequencies and a TDMA receiving apparatus thereof.
2. Description of the Related Art
Conventionally, a base station corresponding to TDMA (Time Division Multiple Access) communicating method using a fan-shaped beam antenna for sectoring a service area has a plurality of independent TDMA receiving apparatuses that use different transmission/reception radio frequencies for individual fan-shaped divided zone.
Such a sector zone method is mainly used to increase the number of subscriber stations accommodated in the service area of the base station with the TDMA receiving apparatuses having the communication capacity for a predetermined number of subscribers and radio frequency bands thereof.
Umeda et al., Japanese Patent Laid-Open Publication No. 3-143123, Ohya, Japanese Patent Laid-Open Publication No. 3-206743, and Nakayama et al., "26 GHz BAND DIGITAL SUBSCRIBER RADIO SYSTEM (26SS-Di) FOR HIGH-SPEED DIGITAL COMMUNICATIONS," IEEE ICC (International Conference on Communication), 1985. 23. 1. 1 (p. 729)--23. 1. 6 (P. 734) disclose examples of the conventional TDMA communicating method and the TDMA receiving apparatus. Japanese Patent Laid-Open Publication No. 3-143123 shows a random access method of a mobile communication system. In this system, when a random access is performed between a base station and each of mobile stations, the signal receive state of each mobile station is detected. Corresponding to the detected result, information that represents a transmission prohibit state or a transmission enable state is sent. Corresponding to the information, the signal transmission is controlled. Japanese Patent Laid-Open Publication No. 3-206743 shows a sector control system. In this system, the control channel zone of a radio base station has a non-directional control channel zone. The control channel zone is divided into a plurality of sectors. Speech channels are controlled for individual sectors. When an originating call or a terminating call is connected to/from a speech channel, a sector of a mobile station to be connected is determined by a radio loop check. Nakayama et al. shows a TDMA communicating method used in a 26 GHZ band digital subscriber radio system.
In the conventional sector zone type TDMA communicating method, independent frames are formed for respective divided zones with different radio frequencies. Considering only the receiving portion of the TDMA communicating apparatus, as shown in FIG. 1, receiving antennas for three fan-shaped divided zones are provided. Outdoor units and indoor units corresponding to the receiving antennas are independently provided. The structure of each of the three independent TDMA receiving apparatuses is the same. Only the receiving radio frequencies of the TDMA receiving apparatuses are different from each other. Thus, in the following description, the structure of only the first TDMA receiving apparatus will be described.
First, a receiving antenna 3a receives a radio frequency burst signal transmitted in the TDMA method by a subscriber station in a particular fan-shaped divided zone. The received signal is sent to an outdoor unit 2a disposed adjacent to the receiving antenna 3a. In the outdoor unit 2a, the received radio frequency signal is amplified and the received frequency is converted into a first intermediate frequency. The resultant signal is sent to a unit connecting cable 4a. The unit connecting cable 4a is connected to an indoor unit 1a so as to send the first intermediate frequency signal from the outdoor unit 2a to the indoor unit 1a. In the indoor unit 1a, a demodulated signal is restored from the first intermediate frequency signal. Moreover, in the indoor unit 1a, information signals of individual subscribers are obtained by synchronizing the TDMA frame.
The outdoor unit 2a has a low noise amplifier (LNA) 5a and a first frequency converter 6a. The low noise amplifier 5a amplifies a received radio frequency signal having weak power at a predetermined amplification factor with a small internal noise. The first frequency converter 6a converts the amplified signal into a first intermediate frequency signal so as to reduce a transmission loss of the signal sent from the unit connecting cable 4a to the indoor unit 1a.
The indoor unit 1a has a second frequency converter 7a, a demodulating circuit 8a, and a TDMA control circuit 9a. The second frequency converter 7a converts the first intermediate frequency signal into a second intermediate frequency signal so as to demodulate the first intermediate frequency signal supplied from the outdoor unit 2a. The demodulating circuit 8a inputs the second intermediate frequency signal and outputs a TDMA signal as a demodulated signal of the second intermediate frequency signal. The TDMA control circuit 9a inputs the TDMA signal, synchronizes the frames of the TDMA signal, and obtains information data signals of individual subscribers that have been time-division multiplexed.
Since the TDMA receiving apparatuses independently operate, the modulated signals that are input to the demodulating circuits 8a, 8b, and 8c are not always the same frequency modulation signal. Thus, the second intermediate frequencies converted by the second frequency converters 7a, 7b, and 7c are not always the same frequency.
As described above, the number of TDMA receiving apparatuses disposed in the base station should accord with the number of fan-shaped divided zones as outdoor units and indoor units.
Therefore, the following problem arises. In the TDMA receiving apparatuses of the base station corresponding to the conventional sector zone type TDMA communicating method, seperate demodulating circuits and TDMA control circuits are required for each individual divided zone. Thus, the hardware scale increases proportional to the number of divided zones, unlike the structure in which the service area is not sectored.
This is because TDMA signals with different radio frequencies sent from a plurality of divided zones corresponding to TDMA synchronous frames are different in individual divided zones.